Abstract
The temperature dependence of coherent carrier transport in quantum cascade lasers (QCLs) is studied in this paper. It was found that coherent carrier transport in QCLs decreases as the temperature increases because the coherence between the injector and active region energy levels decays at a faster rate with increasing temperature. Calculations show that the coherence time decreases by at least a factor of two as the temperature increases from 100 K to room temperature. Electron transport from the injector regions into the active regions and vice versa is a highly coherent process that becomes less efficient with decreasing coherence time and hence becomes less efficient with increasing temperature. As a consequence, when the temperature increases, the population of the upper lasing levels in active regions decreases, the population of the lower lasing levels increases and performance suffers.
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GENERAL SCIENTIFIC SUMMARY Introduction and background. Quantum cascade lasers (QCLs) are an important example of quantum engineering. In the past, these lasers have been designed using primarily the rate equations, which do not take into account quantum coherence. It is known, however, that quantum coherence plays a crucial role in the electron transport and hence the device performance.
Main results. In this work, we develop a density matrix model that includes quantum coherence effects, and we use it to study the impact of temperature changes on the electron transport. We study a realistic QCL structure, with the full set of levels. We find that the rate of electron transitions from the injector regions to the active regions and vice versa depend strongly on the strength of the quantum coherence. We also find that the quantum coherence time in this realistic structure decreases by more than a factor of two as the temperature rises from 100K to room temperature. As a consequence, the electron transport decreases, which can impact significantly on the QCL performance.
Wider implications. Quantum coherence effects should be taken into account when QCLs are designed. We have shown that it is computationally feasible to do so with realistic QCL structures, and we have demonstrated the practical importance by showing that an increase in temperature leads to a decrease in the quantum coherence that will also decrease the electron transport and hence the QCL performance.
Figure. Schematic illustration of the electron transport model. We show how the carrier density of the upper lasing level is related to other levels in the system. The blue straight arrows represent incoherent scattering mechanisms. The red wavy arrows indicate coherent carrier transport. We use double arrows to signify that the carrier transport can be in either direction. Here we show three levels in each of the injector regions and in the active region for illustration. In this work, we considered a realistic structure with a much larger number of levels.